Basics in micro-hydro

What is Micro-hydro? Micro-hydro is fundamentally a small scale version of a power conversion installation using water. People have been tapping the energy in flowing water for centuries, first for mechanical power, and, in the last hundred years, for electricity.

Early applications included milling, pumping, and driving machinery. Unlike wind and sun, the right hydro resource can be available 24 hours a day, 365 days a year. This allowed pioneers to run irrigation pumps and grain mills, and allows people today to make clean, renewable electricity at a reasonable cost.

Hydro power works through the combination of water flow and vertical drop (commonly called “head”). Vertical drop creates pressure, and the continuous flow of water in a hydro system gives us an ongoing source of pressurized liquid energy. Pressurized, flowing water is a very dense resource, and hydro-electric systems convert a very large percentage of the available energy into electricity because the resource is captive in a pipe or flume.

There are a wide range of small hydro turbine types to suit the head and flow of your application. Large wooden overshot and undershot wheels tend to be less efficient for generating electricity, though they may be appropriate for mechanical work. For electricity generation, systems can be divided into “low head” and “high head.”

Getting a head

Low-head systems may have less than 2 meters of vertical drop—sometimes they may have only 25/50 cm. In this case, most or all of the water in a small stream will run through the turbine to maximize output. The runner (the part of the turbine that receives the water and turns its energy into rotation in a shaft) for low-head turbines may be a Turgo or Francis type. These systems typically have short pipelines or sluice ways that then allow the water to drop through the runner.

High-head systems may be defined as any site with more than 3 meters of head. Common runners are Turgos on the low end, and the most common, Pelton, for medium to high heads. These systems may have hundreds of feet of pipeline to develop the head (pressure), with the water delivered to the runner via multiple nozzles.

Within this lies the understanding that systems with low vertical drop (head) need more flow to generate the same amount of energy. Typically, low-head systems will have high flow, and high-head systems will have lower flow.

Basic components of a small hydro-electric system.

Diversion and intake screen—Directs water from the stream or river into the pipe or channel

The device used to capture the energy of the flowing water is the turbine. There are many different types of turbines, but they can be broken down into two different groups – impulse and reaction.

Impulse turbines such as a pelton wheel or a turgo work best when there is a relatively small amount of flow but a relatively large amount of head. The high pressure water stream hits the turbine paddle or spoon, forcing it to turn. The spoons of a turgo and pelton turbine are curved, so the water doesn’t just hit a paddle and fall away, the water actually does a 180 degree turn. The extra force of causing the water to completely change directions makes the turbine spin even faster.

Reaction Turbines are best suited in low head, high flow situations. Impellers and propellers are both reaction turbines. They better capture the energy of the water when there is not enough head to create a high pressure water jet.

The turbine is attached to a generator, which converts the energy of the spinning turbine into electricity. There are many types of generators, but they all work using magnets. As the turbine spins the generator shaft, it causes magnets to spin inside the generator. The magnets spin around coils of wire. The motion of the magnets causes the electrons in the nearby wire to move back and forth or around; and moving electrons is electricity!

If possible we would suggest the use of permanent magnet alternators, also known as three phase brush less motors. This style of generator is extremely reliable because they have no brushes to burn out. By using permanent magnets instead of electro-magnets (like in a car alternator) the generator is extremely efficient. These generators are “three phase” which means they have three wires coming out of them, and the electricity flows back and forth through the three wires to each other.

A special rectifier circuit takes all of this movement and directs it in all one direction; it changes the three wires with alternating current to two wires with direct curent. The two wires will then charge a battery or power an inverter.

The three phase generator is extremely efficient, three phase AC power also travels very well, so if the generator is a long way away from the cabin or whatever you are powering, smaller diameter wires can be used with a minimal loss of power due to line resistance.